When a pharmaceutical composition is applied to patients, it is necessary to guarantee its efficacy and safety, as well as its specificity corresponding to each purpose.
Because of this, great concern has been directed toward the development of a system in which a pharmaceutical composition is designed in such a dosage form that a drug of interest is delivered to a target site for a necessary period of time in a required amount.
In order to satisfy such a requirement, sustained release preparations which can give prolonged duration of action of a drug by controlling the releasing rate of the drug from the preparation have already been developed and put into practical use. In addition, other types of pharmaceutical compositions which can control commencement of the release of drugs have been proposed in recent years, such as a preparation in which a drug is released when the coat membrane of the preparation is disrupted due to swelling of a water swelling material (JP-A-62-30709 (corresponding to U.S. Pat. No. 4,871,549) and JP-A-4-338323; the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a preparation in which a water repellent salt such as magnesium stearate, calcium stearate or the like fatty acid metal salt and an acrylic polymer are used in its coat membrane in order to give the preparation a lag time before the release of its ingredients (JP-A-4-235123 (corresponding to U.S. Pat. No. 5,137,733)) and a preparation in which mutual interaction between Eudragit RS (manufactured by Rohm Pharma GMBH) and an organic acid is applied (Abstract of Papers, 7th Annual Meeting of The Japanese Society of Pharmacy, p.84, 1991).
However, since drugs are produced with various purposes, development of a pharmaceutical composition having various drug release mechanisms which can respond to these purposes has been called for in the field of medicine.